This application claims the priority of German application number 196 47 963.0, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process and apparatus for making holes on the circumference of a hollow section situated in an internal high pressure forming tool.
A process and apparatus of this type are disclosed in German Patent Document DE 195 06 067 C1. There, the wall of a hollow section inserted into an internal high pressure forming tool is acted upon by a punch (which is integrated into the forming tool), from the outside against an internal pressure existing in the hollow section which is lower than the forming pressure customary for the forming of a hollow blank by means of internal high pressure forming. The plate-shaped plane punch penetrates into the wall of the hollow section, and the displaced flowable section material accumulates essentially uniformly on the interior of the hollow section. By means of the punch, an inward penetration or introverting of the hollow section is therefore achieved, whose edge is constructed of the stamping edge or the circumference of the punch. In this case, a cold solidification of the hollow section material takes place, after which, when the punch is withdrawn, the introverting is again pressed to the outside by the now rising internal pressure. The marginal edge of the introverting formed on the punch side in this case forms a shearing edge of the hollow section material for the bottom of the introverting surrounded by it. As a result, at the site of the shearing edge, a surrounding desired breaking point is formed for the hollow section so that the separation of the mentioned bottom or of the later hole slug starts even before the actual cutting-out by means of the cutting edge constructed on the edge of the guide bore of the punch on the die. The cutting-out is therefore facilitated by the preceding separating operation at a defined point.
In the construction of the introverting and of the desired breaking point, the hollow section wall in the area of the future hole edge will be depressed into the interior by the action of the punch, because the internal high pressure is lower than a forming pressure. Although the introverting is pressed out to a certain degree by the rising internal pressure in the hollow section, major sink radii will remain after the generating of the hole which, in the case of different applications, are particularly undesirable, for example because of the lower distribution possibility of the Hertzian pressure and of the smaller support surface during the fastening of add-on parts on the hollow section.
To avoid such deformation on the exterior circumference of the section, a recalibration of the hollow section by means of a forming pressure would be required. Because of the separating operation which starts before the actual cutting-out and the subsequent final cutting-out of the hole slug, a hole is, however, generated before the recalibrating, after which pressure between the hollow section interior and the space adjoining the hole is compensated outside the hollow section. A calibration of the hollow section is therefore no longer possible in the area of the hole, and the elimination of these radii cannot easily be achieved.
Furthermore, a high-expenditure pressure control is required in order to obtain a suitable process pressure in each phase of the separating process, because different pressure levels are provided for each phase. Also, for a final and clean cutting-out, the cutting edge on the tool die must be very precise and sharp, which in the manufacture of tools leads to significant expenditures. Moreover, the cutting edge will become dull after a certain operating time, because of the multiple action upon hollow sections during mass production. Thus, the sharpness of the cutting edge, on the one hand, must be examined in servicing intervals, and on the other hand, it must be reground if it has become unacceptably dull.
The servicing expenditures are considerable and high, in which case, the forming tool, which was produced at high cost itself, will become useless after some time because of an excessive wear as the result of abrasion. In this case, the hole diameter necessarily becomes larger, which is undesirable when narrow tolerances are indicated in the manufacturing and which considerably impairs the guiding of the acting punch. The latter results in a manufacturing of the desired breaking point which differs from one hollow section to the next, which has an unfavorable effect on the quality and the exact reproducibility of the hole production.
It is an object of the present invention to provide a process and apparatus of the above-mentioned type which can produce holes inside the internal high pressure forming tool on the circumference of hollow sections in a simple and reliable manner.
Another object of the invention is to provide a device with a high service life, which maintains the shaping of the outer circumference of the hollow section to be provided with a hole.
These and other objects and advantages are achieved by the method and apparatus according to the invention, in which a desired breaking point is created in the form of a thin area. The latter is dimensioned so that, when the punch is withdrawn, the hollow section material surrounded by the thin area (thus the hole slug) is torn out in a simple manner, driven by the internal high pressure corresponding to the forming pressure during the internal high pressure forming. The internal high pressure forming tool has no other part in the separating process of the hole slug, and is therefore protected from damage by the hollow section on the edge of the guide bore of the punch, so that the service life of the tool is not limited. At the same time, the reliability of the process is increased, because the guide bore of the punch is not subjected to wear on the marginal edge changing into the sinking; therefore no deviations occur during the construction of the desired breaking point. In this case, the production of the hole is almost without tolerances and can be reproduced with accurate dimensions. Only the punches are subjected to wear, which is nevertheless low because they must only penetrate into the hollow section, and not cut through it. In addition, the punch can be easily exchanged.
Because the hole producing operation according to the invention takes place at a uniform pressure level, only slight demands are made on the pressure control. The height of the pressure level. corresponds to the internal high pressure forming pressure during the profiling of a hollow blank or during the calibrating of the hollow section. As a result, the hollow section material around the edge of the hole to be produced is always pressed against the wall of the sinking even during the penetration by the punch so that sink radii produced by the denting-in of the hollow section wall are avoided and thus the outer shape of the hollow section is maintained.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.